1. Field of Invention
The present invention relates in general to an oscillometric non-invasive method and apparatus for measuring blood pressure, and more particularly to a method and apparatus which perform a step-by-step inflation and deflation of a pressure cuff for a comfortable systolic and diastolic pressure determination.
2. Description of Related Art
The heart of a human being is like a pump, and the muscles of the heart periodically contract to force blood through the arteries of the human. As a result, irregularly-shaped pressure pulses exist in theses arteries and cause them to flex or oscillate. The base line pressure for these pulses is known as the diastolic pressure and the peak pressure for these pulses is known as the systolic pressure. A further pressure value, known as the xe2x80x9cmean arterial pressurexe2x80x9d (MAP), represents a time-weighted average of the blood pressure.
In the past, various techniques and devices have been used for measuring one or more of these blood pressure values. The most common method involves applying a pressure cuff about the upper arm of the human and inflating it so as to stop the flow of blood in the brachial artery. The pressure is then slowly relieved while a stethoscope is used on the distal portion of the artery to listen for pulsating sounds, known as Kortotkoff sounds, that accompany the reestablishment of blood flow in the artery. As the pressure in the cuff is reduced further, the Korotkoff sounds eventually disappear. The cuff pressure at which the Korotkoff sounds first appear during deflation of the cuff is a measure of the systolic pressure and the pressure at which these sounds disappear is a measure of the diastolic pressure. This method of blood pressure detection is generally known as the xe2x80x9causcultatory methodxe2x80x9d.
Various devices are well known in the prior art for automatically performing blood pressure measurement by the auscultatory method. These devices employ a pump to automatically inflate a pressure cuff and a microphone to convert the Korotkoff sounds into electrical signals which are easily detected by various types of circuits. Other techniques have also been used to detect blood pressure from outside the subject""s body, e.g., via Doppler shifts in ultrasonic waves reflected by the artery wall. In addition, there are intrusive devices that are inserted directly into the blood vessels for measurement of the pressure. However, the most commonly used method for measuring blood pressure, other than the auscultatory method, is the xe2x80x9coscillometric methodxe2x80x9d.
The oscillometric technique is based on the fact that the pumping of blood through the arteries by the heart causes the arteries to flex. Even in the area adjacent to or within a pressure cuff applied to the arm of a human, these pressure variations exist. In fact, the pressure variations will pass from the artery through the arm of the human with attenuation and into the pressure cuff itself. While these pressure variations are small compared to the typical pressure applied by the cuff, they are nevertheless detectable by a transducer located to measure the pressure within the cuff. It has been found that these pulses, called xe2x80x9ccomplexes,xe2x80x9d have a peak-to-peak amplitude which is minimal for applied cuff pressures above the systolic pressure and-below the diastolic pressure. The amplitude of these complexes, however, rises to a maximum value. Physiologically, the cuff pressure at this maximum value approximates the MAP. It has further been found that the complex amplitudes of cuff pressures equivalent to the systolic and diastolic pressures have a fixed relationship to this maximum value. Thus, the oscillometric method is based on measurements of detected complex amplitudes at various cuff pressures.
However, the above-mentioned oscillometric method has drawbacks. One is that the inflation pressure of the cuff must to be pumped much higher than the normal systolic one. For example, the pressure is generally about 180 mmHg to 240 mmHg for ensuring the detection of the blood pressure value in specific cases of high blood pressure. Users thus always suffer from uncomfortable measurement while the cuff is inflated for a period of time. The continuous contraction of the cuff will cause pain, especially for obese people, and even further harm. On the other hand, the conventional measuring technique is ineffective because the normal systolic pressure is around 130 mmHg and the much higher inflation pressure is needed in case of an exception.
Therefore, it is an object of the present invention to provide a method and an apparatus for performing comfortably and accurately oscillometric blood-pressure measurement.
According to the above object, the blood pressure determining method includes, (a) applying pressure to occlude the artery of a subject; (b) reducing the pressure to permit an increasing flow through the progressively less occluded artery; (c) monitoring arterial counterpressure oscillations at each pressure reducing step; (d) converting the counterpressure oscillations to voltage signals; (e) processing the voltage signals into a sequence of peak amplitudes to obtain a curve; (f) repeating the steps from applying pressure to obtaining a curve until at least two curves, enough for curve fitting an oscillometric envelope, are obtained, and a highest artery occluded pressure is higher than a predetermined value plus a mean arterial pressure determined by one of the curves; and (g) computing a systolic and a diastolic pressure based upon the envelope.
In another aspect, the blood pressure determining apparatus includes, (a) an inflatable and deflatable cuff; (b) controlling means, for controlling cuff pressure so as to perform a step-by-step inflating and deflating of the cuff until a maximum cuff pressure is higher than a predetermined value plus a mean arterial pressure detected for a subject; (c) a monitoring means, for monitoring arterial counterpressure oscillations at each of the cuff pressure deflating processes, a pressure transducer, a processor and a calculator; (d) a pressure transducer means, coupled to the cuff for converting the counterpressure oscillations to voltage signals; (e) a processing means, for processing the voltage signals into a sequence of peak amplitudes at each pressure deflating process, the sequence lying along a curve, and at least two curves, enough for curve fitting an oscillometric envelope, being obtained according the step-by-step inflation-and-deflation process; and (f) a determining means, for determining a systolic and a diastolic pressure based upon the envelope.
Wherein the curves includes an ascending curve, a maximum curve and a descending curve in a graph of oscillation amplitudes. The predetermined value is about 20 mmHg to about 40 mmHg.
The scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.